1. Field of the Invention
The present invention relates to processes for reducing or sealing porosity and filling voids in spray deposited articles, and also to articles formed by such processes.
2. State of Art
Processes for forming articles by means of molten metallic spray deposition techniques (sprayforming) are well known and described, for example, in GB-A-1255862 and WO-A-95/12473. In order to control distortion in sprayed metal deposits it has been proposed to tailor the spraying conditions to control or "balance" the various stresses set up within the cooling deposit. This is particularly the case for crystalline phase change materials such as steels, where the deposition conditions may be tailored to ensure a phase change within the deposited material giving a stress relieving volume change. Such techniques are described in WO-A-96/09421.
A major problem with such techniques is that it is often necessary, in order to ensure the required conditions for stress control, to deposit the material at a lower spray temperature than would normally be chosen for sprayforming applications in which stress control is less critical (for example in depositing thin coatings). Because of the relatively low spraying temperature (preferably below 250-300 Celsius for steels) the sprayform splats do not coalesce particularly well upon deposition which results in a deposit of relatively high porosity; this is a particular problem where the porosity is interconnected. Interconnected porosity occurs where spaced regions within the deposited material are connected by a network of porosity which allows gas or liquid to permeate or percolate between the spaced regions. It is a particular problem where interconnected porosity communicates with a region of porosity at a surface of the deposit (such as a working surface of a mould or die), or with cavities or bores intended to carry or retain fluids (such as coolant channels provided in the article) because leakage may occur. This would be important, for example where the article is a plastic injection moulding tool provided with internal cooling channels, or where leakage of vacuum could occur for tooling used in autoclave applications (for example, in aerospace tooling for making composite lay-ups).
Furthermore, any significant porosity at the working surface of a mould tool or die results in a poor surface finish when the tool is subsequently polished.
As mentioned above these problems of porosity (and also the setting up of internal stresses) are inherently associated with various sprayforming techniques where material is deposited at a relatively low temperature for various reasons that may be desirable. This is because of the nature of the process, in which the deposit is built up from a multiplicity of molten splats of material comprising molten droplets which cool upon impact with a substrate or earlier deposited splats. Such problems do not typically occur with other techniques in metallurgy and other fields, such as for example plasma spraying or flame spraying techniques in which the material sprayed is at substantially higher temperatures (typically 500-800 Celsius for steels).
A further problem associated with sprayforming techniques is "shadowing" which is prone to occur when sprayed material is prevented from impinging upon a particular surface portion by instead impinging upon a "masking" portion of either previously deposited material or the pattern or substrate upon which the deposit is being built up. Such "shadowing" effects frequently result in voids being formed in the interior of a sprayed deposit.